A fault current limiter (FCL) is a device that limits fault currents, typically in a power system. Various types of FCLs have been developed over the last several decades, including superconducting fault current limiters (SCFCLs), solid state fault current limiters, inductive fault current limiters, as well as other varieties that are well known in the art. A power system in which a FCL is implemented may include generation, transmission, and distribution networks that generate and deliver power to various industrial, commercial, and/or residential electrical loads.
A fault current is an abnormal current in an electrical system that may result from a fault in the system, such as a short circuit. A fault current may arise in a system due to any number of events or failures, such as power lines or other system components being damaged by severe weather (e.g. lightning strikes). When such a fault occurs, a large load can instantaneously appear in the circuit. In response, the network delivers a large amount of current (i.e. fault current) to the fault load. This surge of current is undesirable because it can damage the load which may be, for example, the network itself or equipment connected to the network.
FIG. 1A depicts a circuit diagram of an exemplary prior art power system 100 having a FCL with a conventional fixed shunt 114 illustrated in a steady state condition. The exemplary power system 100 includes an AC power source 102, a circuit breaker 108 which is normally closed, and various loads 110. Under steady state conditions, the AC power source 102 provides power to the loads 110. The circuit breaker 108 is closed and 100% of the current from the AC power source flows through conductor 103, the FCL 106, and the conductor 105 to the loads 110, as illustrated by arrow 150.
FIG. 1B depicts the circuit diagram of FIG. 1A illustrated in a fault condition before the circuit breaker 108 has opened. For example, a fault condition may occur at location 112 represented by the inadvertent path to ground. In response to the fault, the AC power source 102 attempts to deliver a large amount of fault current to the fault load and the FCL 106 exhibits a resistance much larger than the fixed shunt 114. For example, if the FCL is a superconducting FCL (“SCFCL”) having a superconductor that exhibits essentially zero resistance in a superconducting, steady state condition, the fault current causes the superconductor to quench and thereby exhibit a resistance much larger than that of the fixed shunt 114. Since the resistance of the FCL is much larger, the fault current represented by arrow 152 is commutated into the fixed shunt 114. The fixed shunt 114 limits the fault current to an acceptable level by reducing the peak-to-peak amplitude of the fault current before the circuit breaker 108 can open. A conventional circuit breaker 108 typically takes 2 to 5 cycles of a conventional 60 Hz frequency before opening. During a post fault time interval, the circuit breaker 108 opens and no current is provided to the loads 110 either through the FCL or the fixed shunt 114.
Although fixed shunt FCL systems such as the one described above can be very effective for limiting fault currents, a significant drawback of such systems is that the FCL must be configured to carry all of the anticipated steady state current of the circuit during normal operation, as described above with reference to FIG. 1A. In high current applications this generally requires a FCL having a large physical footprint and high energy consumption. For example, in the case of a SCFCL, the FCL will include a superconductor housed in a cryogenic tank (cryostat). To operate at a nearly zero impedance, superconducting state, the superconductor must be operated below its critical temperature, critical current density, and critical magnetic field. If any one of these three levels is exceeded, the superconductor quenches from its superconducting state to a normal state and exhibits resistance much larger than the resistance of the fixed shunt 114. To maintain the superconductor at a temperature below its critical temperature, a refrigeration system delivers a cryogenic cooling fluid to the cryostat. Accordingly, the quantity of the superconductor material, as well as the capacity of the associated cooling system to maintain the superconductor below its critical temperature, must be sufficient to accommodate all of the steady state current in the system. This may require significant equipment and energy costs. In addition, the physical size of a SCFCL required for a particular application can make installation at the application site difficult or impractical. Similar challenges exist in systems that employ solid state fault current limiters, which employ large numbers of parallel components, exhibit high power losses, and require large and costly cooling systems for handling high system load currents. It is with respect to these and other considerations that the present improvements have been needed.